In my copending application, Ser. No. 281,405, filed on Aug. 17, 1972, now abandoned, for "Improved Electrostatic Precipitation", I have disclosed a procedure for the electrostatic precipitation of particulates entrained in a stream of gas between corona electrodes and collecting electrodes in which an underlying unidirectional field which serves to charge the particulates and transport them out of the stream is made relatively uniform, thereby allowing the creation of relatively high electric field intensities, while the corona which is required to yield ions to charge the entrained particulates is provided by a high, repetitively pulsed, electric field between the corona electrodes and the collecting electrodes. Such a system separates the charge and transport function from the function of production of charge carriers by corona discharge.
A number of other advantages are also comprehended by such systems:
(1) Presently existing electrostatic precipitators can be adapted to such systems without the necessity of major changes in the installation since the electrical circuitry may be quite simple. PA1 (2) The wire breakage rates of such systems may be smaller than those of presently existing electrostatic precipitators since such systems allow corona electrodes of greater cross-sectional area than that acceptable in presently existing precipitators to be used. PA1 (3) In such systems the pulsed field can be chosen sufficiently high that corona current is assured under virtually all operating conditions, thereby alleviating the very sensitive nature of conventional system electrodes to contamination, and the operative range of the dc field is greatly increased. PA1 (4) Such systems also allow the average value of the corona current to be closely regulated independently of the dc field by adjusting the superimposed pulse voltage, pulse width, or pulse repetition rate. Thus, "back corona" can be controlled, and the minimum level adequate to charge the particulates close to their equilibrium state need not be significantly exceeded.
I have found, however, that such systems, especially converted conventional precipitators, may be costly to operate. A typical electric utility system with which the precipitator of the present invention might be used might be one with an electric power output of 7 megawatts. Assuming, for example, that the power is generated by burning coal, the products of combustion might result in a typical case in a gas flow of 50,000 cubic feet per minute. In order to clean this gas flow a typical total anode collecting area would be 20,000 square feet, and with typical wire-to-plate spacing the capacitance of the precipitator would be 100 nanofarads. A conventional rectified unfiltered dc system would have a total current of 1 ampere and a dc voltage of 70 kilovolts, thus resulting in a total power consumption of about 70 kilowatts. If the improvement described in my co-pending application Ser. No. 281,405, referred to above, were used for the conversion of such a conventional system to a pulsed system, the delivery of a pulse amplitude of 70 kilovolts thereto would require an energy of 735 joules per pulse to superimpose a single pulse onto the dc level. If one further assumes a pulse width of 100 nanoseconds, which I have suggested as typical for such systems, and ionization parameters such that a repetition frequency on the order of 10.sup.4 pulses per second is required to produce the necessary corona current, a total power consumption on the order of 7.35 MW is the result. Even 10.sup.3 pulses per second yields power consumption figures of 735 KW.
It will be noted that this power consumption has nothing to do with the useful power consumed in particulate removal, but is solely the reactive power required to charge the capacitance of the precipitator for purposes of the pulse. If sharp pulses are to be produced, as is necessary in the above improvement, it is necessary that the charge applied to the capacitance for purposes of the pulse must somehow be dissipated between pulses, and this is where the power loss occurs. It will be noted that in the above example at a repetition frequency of 10.sup.4 pulses per second the reactive power consumption exceeds that of the utility plant itself, and even at 10.sup.3 pulses per second the reactive power loss is over 10 percent.
The power discrepancy is clear. At present rates, the energy costs alone for a system such as the one described above would be around $160,000 per year. This figure is comparable to the present cost of the electrical portion of a conventional precipitator, and electric rates are rising steadily. Clearly, means are needed to reduce power requirements of improved systems if there are ever to be practical alternatives to conventional models, much less improvements thereon.
A reduction of pulse amplitude is one possible approach to this problem. This approach is unattractive, however, since it leads to a configuration and operation only slightly different from conventional dc charged precipitators, and it significantly reduces the usefulness and availability of the beneficial factor of controllable corona current. A reduction of pulse repetition frequency is also unattractive. It is possible to vary this parameter somewhat, but it will be desirable to provide sufficient charge carriers to charge the particulates close to their equilibrium value in a time which is short in comparison with the particle crossing time. Thus, given a representative drift velocity of 70 cm/sec, Cf. J. W. Parkington, M. S. Lawrie-Walker, "Attainment of High Precipitation Efficiencies on Fine and Sub-Micron Dusts and Fumes," LA PHYSIQUE DES FORCES ELECTROSTATIQUE ET LEURS APPLICATIONS, pp. 351-362, Grenable (1960), and an average perpendicular travel distance of 7 cm, for example, a repetition frequency far in excess of 10 pulses per second (pps) is indicated. One hundred pulses per particle crossing time, which would not be unusual, suggests 1000 pulses per second as a typical value for this parameter. Experiments with each specific system and particulate are necessary to determine optimal repetition frequency.
In addition to the excessive power requirements of pulsing a precipitator in the manner described above, in which the pulse voltage is applied to all the precipitator's wires simultaneously, a further problem is the difficulty in producing a short-rise time of the pulse. A typical inductance between the pulser and the cathode structure would be of the order of 1 microhenry. If one assumes therefore an inductance of one microhenry and if one considers a precipitator capacitance of 100 nf per pulser one arrives at a pulse rise time of approximately one-half microsecond. This time is already too long to take advantage of the increased hold-off strength of gases for short pulses. Differently stated, the excessive time required to reach the peak of the pulse effectively increases the pulse length obtainable.